Burner and method of operating it to control the production of nitrogen oxides

ABSTRACT

In the operation of a burner using gaseous fuel the production of nitrogen oxides is limited and controlled by providing successive stages of combustion and by holding down the temperature of combustion. This is accomplished by introducing the fuel in two streams of unequal volume and comprising respectively more and less than the stoichiometric percentage of air, igniting the streams in sequence, thus effecting first and second stages of combustion, and mixing and consuming the excess fuel and air of said streams respectively, thus effecting a third stage of combustion.

' United States Patent [191 Schwedersky [11] 3,729,285 45 Apr. '24, 197341 BURNER AND METHOD OF OPERATING IT TO CONTROL THE PRODUCTION OFNITROGEN OXIDES [76] Inventor: George H. Schwedersky, 276 Main Street,Westport, Conn. 06680 [22] Filed: May 22, 1972 [21] Appl. No.: 255,532

[52] US. Cl. ..431/8, 431/351, 431/284 [51] Int. Cl. ..F23c 7/00 [58]Field of Search ..431/2, 10, 187, 174,

[56] References Cited UNITED STATES PATENTS 3,048,131 8/1962 Hardgrave..1 10/72 3,610,537 10/1971 Nakagawa et a1. ...239/424.5 3,308,8693/1967 Livingston 431/187 3,163,203 12/1964 lhlenfield ..43l/284 XPrimary ExaminerWilliam F. ODea Assistant Examiner-William C. AndersonAtt0rneylohn W. Hoag [57] ABSTRACT In the operation of a burner usinggaseous fuel the production of nitrogen Oxides is limited and controlledby providing successive stages of combustion and by holding down thetemperature of combustion. This is accomplished by introducing the fuelin two streams of unequal volume and comprising respectively more andless than the stoichiometric percentage of air, igniting the streams insequence, thus effecting first and second stages of combustion, andmixing and consuming the excess fuel and air of said streamsrespectively, thus effecting a third stage of combustion.

5 Claims, 2 Drawing Figures Patented April 24, 1973 3,729,285

lsT SIR/99M BURNER AND METHOD OF OPERATING IT TO CONTROL THE PRODUCTIONOF NITROGEN OXIDES FIELD OF THE INVENTION This invention relates to aburner of the kind in which propane gas or other gaseous fuel iscombined with air and burned to provide heat, as for example to providesteam in an electric power generating plant. The invention isparticularly directed to the method of operating such a burner to limitand control the amount of nitrogen oxides which are produced whileobtaining substantially complete and therefor efficient combustion.

SUMMARY OF THE DISCLOSURE In accordance with this invention theproduction of nitrogen oxides is kept at a low level by providingsuccessive stages of combustion, each occurring at a temperaturesubstantially less than would be obtained in a single stage ofcombustion using the same volumeand proportion of gaseous fuel/air.Instead of injecting gaseous fuel and air into the burner to mix andprovide a single stream in which for efficient heating the percentage ofair would be the optimum for obtaining complete combustion of thegaseous fuel, the gaseous fuel and air are introduced so as to form twoseparate streams of different volume and each comprising differentratios of gaseous fuel to air. One of the streams comprises an excess ofair and theother stream comprises an excess of gaseous fuel. One of thestreams is first ignited effecting a first stage of combustion. It thenignites the second stream effecting a second stage of combustion, and athird stage of combustion is provided by the mixing and burning of theexcess gaseous fuel in one of the streams with the excess air in theother of the streams. The temperature created in each stage is less thanwould have been created if the same volumes of gas and air had beenmixed and consumed in a single stage, but the total of heat produced issubstantially the same as would have been produced in a singlecombustion stage.

The invention will be best understood if the following description isread in connection with the drawing in which:

FIG. 1 is a side elevation showing a burner having two sets of gas andair inlets disposed to provide two separate gaseous fuel/air mixturestreams and means for igniting them, and the excess air from one streamand the excess gaseous fuel from the other stream, in sequence providinga cascading three stage combustion, and

FIG. 2 is a vertical section taken on the line 2-2 of FIG. 1.

DESCRIPTION Great difflculty has been experienced in attempting toreduce the amount of nitrogen oxide produced in air-gas burners. It iswell established by experiments that the production of nitrogen oxidesis highest when the mixture of gaseous fuel and air employed is close tothe optimum mixture for combustion, or, in other words close to thepoint where the temperature of combustion is the highest. Reference tothe Proceedings, Conference on Natural Gas Research and Technology,

sponsored by American Gas Association, Inc. and Institute of GasTechnology, held at Chicago, Ill., Feb. 28Mar. 3, 1971, and particularlyto Session IV, Paper 2, and the curve which is shown on page 20 of thatPaper and is identified as FIG. 7., Variation of Nitrogen OxidesConcentration with Stoichiometry for a Premixed Natural Gas/Air Flameconsuming 200 Standard Cubic Feet Per Hour of Gas, indicates that themaximum production of nitrogen oxide occurs when a percent of airslightly in excess of the optimum amount of air for combustion, about107 percent of stoichiometric air, is mixed with the gaseous fuel, andthat the production of nitrogen oxides falls off rapidly when thepercentage of stoichiometric air by volume is further increased, or isdecreased. The curve indicates that when the gas/air mixture burnedcomprises approximately l07 percent of stoichiometric air by volume theaccompanying production of nitrogen oxides is 120 parts per million byvolume, dry basis, whereas when 140 percent of stoichiometric air isemployed the nitrogen oxides concentration falls to approximately partsper million by volume. Similarly the curve indicates that if onlypercent of stoichiometric air by volume is present in the gas/airmixture production of nitrogen oxides falls to approximately 70 partsper million by volume.

In order to reduce the temperature at the time of ignition and therebysubstantially reduce the production of nitrogen oxides I have employedthe information indicated by the said curve to limit and control theproduction of nitrogen oxides and at the same time obtain an efficientand substantially complete combustion. In order to accomplish this,instead of introducing into the burner a single mixture of gas and air,in accordance with my method two mixtures of gaseous fuel and air areintroduced separately into the burner and ignited in sequence. One ofthe gaseous fuel-air mixtures comprises substantially more than percentof stoichiometric air and the other gaseous fuel-air mixture comprisessubstantially less than 100 percent of stoichiometric air. The firstmentioned mixture therefor comprises a substantial excess of air and thesecond gaseous fuel-air mixture comprises a substantial excess ofgaseous fuel. When the two streams are ignited the temperature producedby each will be substantially less than the temperature produced byigniting a single gaseous fuel-air stream having the volume of thecombined two streams, and comprising 100 percent of stoichiometric air.The temperature close to the burner face is therefor greatly reduced,the exact amount of reduction varying to some extent depending upon thegaseous fuel employed.

With a single stream of gaseous fuel/air, if the ratio is the optimumfor obtaining complete combustion, the high flame temperature realizedat the burner will fall off gradually as the gases travel to the flue.In the case of the two streams which are ignited in succession andcomprise the same total volume of gaseous fuel/air as a single stream,the flame temperature initially adjacent the burner face is lowerbecause of the excess air and excess fuel in the two streams, but thetemperature of the gases is sustained by the combustion in the secondand third stages, and that temperature will be substantially the same asthat of the single stream' when the gases of the two streams reach thefurnace flue and therefor the total transfer of heat is essentiallythesame. h j

By properly proportioning' the volume of the two gaseous fuel/airmixtures and the ratio..of gaseous fuel to air in each mixture theexcess of air in one of the mixtures can be'proportioned to the amountof excess fuel in the other of the mixtures so that during the threestages of combustion complete or substantially complete combustion willbe realized and a total production of heat obtained equal to, orsubstantially equal to the amount of heat which can be obtained from asingle stream of gaseous fuel equal in volume to the gaseous fuel in thetwo streams and mixed with lOO percent of stoichiometric air, theessential difference being that by the single flame a flash temperatureapproximating 2,800 F. 3,200 F. would be obtained whereas by employingthree stages of combustion as described above a much lower temperaturenear the burner face is obtained, a temperature which in the case ofnatural gas would be in the neighborhood of 2,l F. 2,400 F. Reference toFIG. 8 on page 21 of Paper 2, Session IV of the aforesaid Proceedings ofthe Conference on Natural Gas Research and Technology shows that closeto the burner face a higher temperature results in a proportionatelygreater concentration of nitrogen oxides measured in parts per millionby volume on a dry basis.

In the embodiment illustrated in the drawings a burner 10 comprises aburner face 12 disposed between air and gaseous fuel supply conduits l4,l6 and 18, and a burner tile chamber 22 which at its far end opens intothe combustion chamber 24 of a furnace or the like indicated generallyby the numeral 26. As shown the burner tile chamber 22 projects into thefurnace combustion chamber 24 and a wall 28 of the furnace extendsaround the burner tile chamber.

Gaseous fuel conduit 14 and the surrounding air conduit l6 communicatewith the interior of the burner tile chamber 22 through orifices and 32respectively in the burner face 12. Gas and air is supplied underpressure through said conduits to mix adjacent the burner face andprovide a first gaseous fuel/air stream. This stream is ignited by asuitable ignition means 36, preferably disposed at the burner face, andit may have the ignition wire 36a extending to it through conduit 14,and the flame is directed into the tubular and perforate choke collar34.

Gaseous fuel conduit 18 and the air conduit 20 communicate with theinterior of the burner tile chamber through the orifices and 42. Gas andair is supplied through conduits 18 and 20 respectively to mix and forma second gaseous fuel/air stream which is concentric with, and spacedradially outwardly of, the first stream. The second stream is notdirected into the choke collar 34 but flows around it, and is ignitedafter the first stream by the flame of the first stream issuing throughthe perforations 35 in the choke collar 34. The excess of fuel in onestream mixes with the excess air in the other stream and is ignitedwithin the merged flame 44 resulting from the sequential ignition ofthe'two streams thus providing a third stage ofignition.

It will be noted that wall 46 separates the two sets of conduits whichsupply the two streams,.and that orifices 40 are inclined to direct thegaseous fuel from conduit 18 to mix with air from conduit 20 and not tobe injected into thechokelcollar 34. The burner tile is shaped so as tobring'the excess fuel into the center stream thus mixing it with theexcess air causing them to burn and thus releaseall. of the heat fromthe fuel supplied in the two streams.

It will be understood that if desired the two streams of gaseousfuel/airmay be premixed before entering the burner.

The conduits 14 and 16 may be employed to provide a constant minimumsupply of gaseous fuel and the conduits 18 and 20 may be controlled toincrease the fuel supply as may be desired.

To illustrate my method more specifically I prefer to use two mixedstreams of gaseous fuel/air which differ in volume by the ratio of oneto three, with a 1 to 1.75 proportion of gas to stoichiometric air inthe smaller stream, and a l to 0.75 proportion of gas to stoichiometricair in the larger stream. This means that in the smaller stream therewill be percent excess air, and in the larger stream there will be a 25percent deficiency of air, or stated oppositely a 25 percent excess ofgas. When these two streams are ignited in sequence all of the gas inthe smaller stream will be ignited and burned close to the burnerleaving approximately 75 percent unconsumed air which will flowoutwardly within the burner tile chamber and the combustion chamber. Allof the air in the second stream will combine with gas in the secondstream and will be ignited and burned a little further from the burnerface, leaving an excess of 25 percent of gas which will flow outwardlywithin the burner tile chamber and the combustion chamber and becomemixed with the excess air from the first stream. Since the volume of thelarger stream is three times as great as that of the first stream therewill be relative volumes of gaseous fuel and air which are the optimumfor complete combustion and this mixture will burn in a third stage ofcombustion which is a continuation of the first and second stages, thethree stages occurring at progressively greater distances from theburner face. Thus in successive stages complete or substantiallycomplete combv ation of the total amount of gaseous fuel and air will beachieved, but the temperature of each stage will be very substantiallyless than in the case of a single stage of combustion employing percentstoichiometric air and the same total volume of gaseous fuel, and thetotal amount of heat produced will be the same or approximately the sameas would have been obtained from a single stage of combustion at a muchhigher initial temperature.

LIST OF PARTS l0 burner 12 burner face 14 gaseous fuel conduit 16 airconduit 18 gaseous fuel conduit 20 air conduit 22 burner tile chamber 24combustion chamber 26 furnace 28 wall of 26 30 orifice for conduit 14 32orifices forfc 'onduit 16 34 choke collar 35 perforations in 34 36ignition means 36a ignition wire 40 orifices for conduit 18 42 orificesfor conduit 44 merged flame 46 wall of burner What I claim is:

1. In the operation ofa burner the steps of,

introducing into the burner two streams of mixed air and fuel in gaseousphase,

controlling the air to fuel ratio of the streams so that one comprisesan excess of stoichiometric air and the other comprises an excess offuel and,

igniting said streams in sequence, and thereafter mixing the excess airfrom one stream with the excess fuel from the other stream and burningthe resulting mixture.

2. The method claimed in claim 1 in which one of said streams is ofgreater volume than the other, and the sum of the percentages'of air inthe two streams approximates the optimum or stoichiometric airpercentage.

3. A burner adapted to provide combustion of gaseous fuel and air insuccessive stages which comprises:

gaseous fuel and air conduits,

a burner tile chamber,

a perforate choke collar within the burner tile chamber,

an ignition means disposed at the burner face,

conduit and orifice meanS for injecting a first stream of mixed gaseousfuel and air into the choke collar within the burner tile chamber, andother conduit and orifice means for injecting a second stream of mixedgaseous fuel and air into the burner tile chamber to pass over andaround the choke collar.

4. The burner claimed in claim 3 in which the said conduits areconcentric and the choke collar is axially aligned with the centerconduit means.

5. In the operation of a burner using a gaseous fuel/air mixture whichwhen burned forms nitrogen oxides, the concentration of the nitrogenoxides being greatest when the volume of air employed is substantiallythe optimum for combustion and therefor when burned has a hightemperature, the step of minimizing the temperature of combustion andthe formation of the nitrogen oxides by providing multistage combustionat successive distances from the burner face said multistage combustionbeing achieved by dividing the gaseous fuel/air supply into two streams,one of the streams comprising excess air and the other stream comprisingexcess gaseous fuel, igniting the said two streams in successive stagesand thereafter mixing and burning the remaining excess air and excessfuel from said streams respectively providing a third stage ofcombustion.

2. The method claimed in claim 1 in which one of said streams is ofgreater volume than the other, and the sum of the percentages of air inthe two streams approximates the optimum or stoichiometric airpercentage.
 3. A burner adapted to provide combustion of gaseous fueland air in successive stages which comprises: gaseous fuel and airconduits, a burner tile chamber, a perforate choke collar within theburner tile chamber, an ignition means disposed at the burner face,conduit and orifice meanS for injecting a first stream of mixed gaseousfuel and air into the choke collar within the burner tile chamber, andother conduit and orifice means for injecting a second stream of mixedgaseous fuel and air into the burner tile chamber to pass over andaround the choke collar.
 4. The burner claimed in claim 3 in which thesaid conduits are concentric and the choke collar is axially alignedwith the center conduit means.
 5. In the operation of a burner using agaseous fuel/air mixture which when burned forms nitrogen oxides, theconcentration of the nitrogen oxides being greatest when the volume ofair employed is substantially the optimum for combustion and thereforwhen burned has a high temperature, the step of minimizing thetemperature of combustion and the formation of the nitrogen oxides byproviding multistage combustion at successive distances from the burnerface said multistage combustion being achieved by dividing the gaseousfuel/air supply into two streams, one of the streams comprising excessair and the other stream comprising excess gaseous fuel, igniting thesaid two streams in successive stages and thereafter mixing and burningthe remaining excess air and excess fuel from said streams respectivelyproviding a third stage of combustion.